Rendering only missing frames on animation
In the Render Settings under Output you will find the overwrite option will do what you want.


  Overwrite - Overwrite existing files when rendering.


What that means is that when overwrite is disabled and blender goes to render frame 12 and the file for frame 12 already exists it will not render that frame and go onto the next one.

For the VSE, if a bad (or empty) file exists for a frame when you add the image sequence to the VSE it will read the updated file for the bad frame/s once you have rendered them.